Patients that are on the following CYP3A4 inhibitors and cannot replace these medications with other equivalent medications for the period of the study: antiarrhythmics (amiodarone, quinidine), neuroleptics (pimozide), sedative/hypnotic agents (midazolam, triazolam), ergot derivatives (dihydroergotamine, ergonovine, ergotamine, methylergonovine), HMG-CoA reductase inhibitors (lovastatin, simvastatin, atorvastatin), rifampin, rifabutin, felodipine, nifedipine, and sildenafil or St. John's wort. Patients whose baseline medication regimen includes 2 or more medications of a class carries the potential for serious side effects, and which must be changed becaused of potential interaction with nelfinavir, they must be stable on the new regimen for 7 days before enrollment.